How does the scope of variables in julia modules work?

Question

Running this code

module mod1
export foo

function foo()
    i=0
    for ii in 1:10
        global i+=ii
    end
    i
end

end

mod1.foo()

gives UndefVarError: i not defined.

It seems that the way to do it is by adding a global before the variable i:

module mod2
export bar

function bar()
    global i=0
    for ii in 1:10
        global i+=ii
    end
    i
end

end

mod2.bar()

This gives: 55

Why does the first method not work? As I understand the for introduces a new scope. Therefore I need the global inside the loop. But why do I also need it outside of the loop?

(I am using julia 1.5.0)

François Févotte François Févotte · Accepted Answer · 2020-10-02T09:33:22

The correct way to write this would be:

module mod1
export foo

function foo()
    i=0
    for ii in 1:10
        i+=ii
    end
    i
end

end

julia> mod1.foo()
55

There are 3 scopes introduced here (from outermost to innermost):

global scope of module mod1
local scope of function foo
local scope of for body

The rules for scoping are explained in the manual. In this particular case: when i is referred to in the for body, we first look for a variable named i defined in that same scope. Since none is found, we look for it in the enclosing scope which is the local scope of foo... and find the variable declared using i=0.

If no variable named i had been found in the local foo scope, we'd have had to look into the enclosing scope, which is a global scope. And in this case, you'd have had to put the global keyword to explicitly allow it (because it has performance implications).

How does the scope of variables in julia modules work?

1 Answers